U.S. patent number 4,397,588 [Application Number 06/227,545] was granted by the patent office on 1983-08-09 for method of constructing a compacted granular or stone column in soil masses and apparatus therefor.
This patent grant is currently assigned to Vibroflotation Foundation Company. Invention is credited to R. Robert Goughnour.
United States Patent |
4,397,588 |
Goughnour |
August 9, 1983 |
Method of constructing a compacted granular or stone column in soil
masses and apparatus therefor
Abstract
Compacted granular or stone columns are constructed in soil to
increase the load-bearing capacities of the native soil. The upper
portion of the compacted granular column is provided with a rigid
central core such that vertical loads imposed on the composite
column are transferred to a deeper level on the compacted column
where the column operates more efficiently. A probe is centrally
penetrated downwardly into the compacted granular column, and the
resulting cavity is filled with cementitious grout to form a solid
core after hardening. The grout may be injected into the cavity so
formed in the compacted column at the bottom of the probe as the
probe is being withdrawn in predetermined quantities metered in
synchronization with the rate of withdrawal of the probe from the
core cavity.
Inventors: |
Goughnour; R. Robert (Allison
Park, PA) |
Assignee: |
Vibroflotation Foundation
Company (Pittsburgh, PA)
|
Family
ID: |
22853519 |
Appl.
No.: |
06/227,545 |
Filed: |
January 23, 1981 |
Current U.S.
Class: |
405/236; 405/232;
405/233; D15/21 |
Current CPC
Class: |
E02D
3/08 (20130101); E02D 27/26 (20130101); E02D
5/36 (20130101) |
Current International
Class: |
E02D
27/26 (20060101); E02D 27/00 (20060101); E02D
3/00 (20060101); E02D 3/08 (20060101); E02D
5/34 (20060101); E02D 5/36 (20060101); E02D
007/00 () |
Field of
Search: |
;405/233,229,231,242,240,236,237 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Soil Improvement-History; Capabilities and Outlook," Report by the
Committee on Placement and Improvement of Soils of the Geotechnical
Engineering Division of the American Society of Civil Engineers,
Feb. 1978..
|
Primary Examiner: Taylor; Dennis L.
Attorney, Agent or Firm: Carothers & Carothers
Claims
I claim:
1. A method of constructing a compacted granular or stone column in
soil to increase load-bearing capacities comprising the steps of
penetrating a probe downwardly into soil to be compacted to a
predetermined depth thereby forming an elongated cavity in the
soil, withdrawing the probe from the soil cavity, backfilling at
least a portion of the cavity with granular material and compacting
the granular material in the cavity to form a compacted column,
penetrating a probe downwardly into the compacted column to a
predetermined depth thereby forming an elongated cavity in the
compacted column, withdrawing the probe from the cavity formed in
the compacted column, and filling the cavity formed in the
compacted column with a cementitious grout to form a solid core
after hardening of the grout.
2. The method of claim 1, wherein the steps of withdrawing the
probe from the cavity formed in the compacted column and filling
the cavity formed in the compacted column with grout are carried
out by simultaneously withdrawing the probe whie injecting grout
into the cavity at the lower end of the probe in predetermined
quantities metered in synchronization with the rate of withdrawal
of the probe from the cavity.
3. The method of claim 2, wherein the step of injecting grout is
carried out by pumping the grout into the cavity formed in the
compacted column with a positive displacement grout pump and
regulating the pump output with the rate of withdrawal of the
probe.
4. The method of claim 3, wherein the pump output is regulated to
continually fill the cavity formed in the compacted column by the
probe with grout as the probe is being withdrawn.
5. The method of claim 3, wherein the pump output is regulated to
continually fill the cavity formed in the compacted column by the
probe with grout under pressure in the column cavity as the probe
is being withdrawn.
6. The method of claim 1, wherein the step of backfilling is
carried out by backfilling at least a portion of the cavity with
fine granular material such as sand as the granular material.
7. The method of claim 1, wherein the step of backfilling is
carried out by backfilling at least a portion of the cavity with
coarse granular material such as crushed stone or gravel as the
granular material.
8. The method of claim 1, wherein at least one of the steps of
penetrating the probe downwardly also includes the step of
laterally vibrating the probe while penetrating it.
9. The method of claim 1, wherein at least one of the steps of
withdrawing the probe also includes the step of laterally vibrating
the probe while withdrawing it.
10. The method of claim 9, wherein the probe is laterally vibrated
during the step of withdrawing it from the cavity formed in the
compacted column and the step of filling the column cavity with
grout is carried out simultaneously with the step of withdrawing
the probe from the compacted cavity.
11. The method of claim 1, wherein the steps of backfilling with
granular material and compacting the granular material to form a
compacted column are carried out by backfilling the soil cavity
with layers of granular material and successively repenetrating
each added layer with a laterally vibrating probe for compaction to
form the compacted column layer by layer.
12. The method of claim 1 or 2, including the step of jetting a
fluid under pressure from the bottom of the probe during at least
one of the steps of penetrating to assist in penetration of the
probe.
13. The method of claim 1 or 2, wherein the step of backfilling of
the soil cavity is carried out to completely fill the soil cavity
with granular material.
14. The method of claim 1 or 2, wherein the step of backfilling of
the soil cavity is carried out to only partially fill the soil
cavity to a predetermined level, and including the step of filling
the remaining top portion of the soil cavity with cementitious
grout on top of the compacted column after the grout core has been
formed therein.
Description
The present invention relates generally to improvements relating to
the treatment of soil masses for foundations and like structures,
and more particularly to the construction of compacted granular or
stone columns in soil masses.
The densification of cohesionless granular soils such as sand with
vibratory equipment is a well known construction procedure to
increase the load-bearing capacities of the soil. One popular
method of densifying such cohesionless soil is the use of vibratory
earth probes in combination with water jetting and flooding. A good
example of this technique is illustrated in Steuerman U.S. Pat. No.
2,718,761. In such a method, the densification of the cohesionless
soil is carried out by the application of mechanical vibration and
simultaneous application of water nullifying effective stresses
which exist between adjacent soil grains. The soil grains in an
unconstrained and unstressed condition, are rearranged to their
densest possible state under the continued application of
vibration, and jetted induced stress reduction. This basic type of
process has been economically applied as a foundation solution
since the latter part of the 1930s, with considerable success
throughout the world.
However, the application of this method in cohesive soils, such as
clays or silts, does not produce the same results. In cohesive
soils, contact forces between individual particles cannot be
eliminated by vibration and, therefore, these soil particles are
not separated, even temporarily, during the same process.
Similarly, in soil such as fine grain silts with low permeability
that exhibit "apparent cohesion," the particles are difficult to
separate by such a vibration process. It is for this reason that
with regard to discussion of the present invention, these fine
grain silts are included in the category of cohesive soils.
Although these afore-mentioned vibration techniques do not
materially improve the consistency of cohesive soils, a variant
method was developed in Germany approximately twenty years ago to
strengthen such soils in situ. This method is generally referred to
as a construction technique termed as the construction of stone
columns, which strengthens cohesive soils to a point where they are
able to sustain considerably larger bearing stresses without
developing detrimental or excessive settlement, or bearing capacity
failure.
Stone columns, as the name implies, are simply vertical columns of
compacted crushed stone, gravel or sand which extend through a
deposit of soft material or soil to be strengthened. A number of
acceptable methods are available for installing such compacted
granular columns or stone columns. An example of such prior art
equipment utilized for this purpose is illustrated in the
afore-mentioned Steuerman patent and in Steuerman U.S. Pat. No.
2,667,749 and in Van Weele U.S. Pat. No. 3,858,398. Other effective
methods are illustrated in Mars U.S. Pat. No. 4,126,007 and in
Ogawa U.S. Pat. No. 3,772,892.
When vibratory probes are utilized, similar to the type illustrated
in the afore-mentioned Steuerman patents, to construct a sand or
stone column, the probe itself generally consists of a 12 to 16
inch diameter hollow cylindrical body which can be, for example, 7
to 15 feet in length and which is connected by a special elastic
coupler to an upper series of follower tubes. Eccentric weights in
the lower part of the probe are driven by an electric or hydraulic
motor operating usually at 3,000 revolutions per minute at 50 Hertz
or at 1,800 revolutions per minute at 60 Hertz to create lateral
vibrations in the probe or vibrations in the horizontal plane. The
total weight of the probe is adjustable by the addition of the
afore-mentioned follower tubes which can be heavy or light and
which can produce a total weight of approximately 4 to 8 tons for a
45 feet long probe. The electric cables and/or hydraulic hoses and
water hoses are connected usually to the uppermost extension tube
and two sets of water outlets or jets are located along the probe
length. The lower set of water jets are located at the probe's
lower tip, and aid in probe penetration, while the upper set of
water jets assist in the removal of displaced cohesive material
which lies within the probe's path. The complete assembly is
usually supported from a commercial crane. Special supporting rigs
have also been developed in the past which can exert a downward
hydraulic thrust to force the probe into the ground. Other
supporting equipment would generally consist of a high
capacity-high pressure water pump, a portable energy source to
provide power for the probe motor and pumps and a front end loader
or other means to feed the required granular backfill material into
the soil cavity formed by the probe.
In order to construct a stone column with this type of probe, the
probe is penetrated into the soft cohesive soil to a predetermined
depth under its own weight and with vibration and assistance of a
jetting fluid. The jetting fluid may be water under pressure or
compressed air. During the penetration process, the soil
immediately surrounding the vibrator is disturbed or remolded to a
nominal extent. When water is used during the jetting process, the
disturbed material is flushed from the hole, however, true
displacement of the in situ soil will occur when compressed air is
utilized as the jetting medium. After penetration to the full
depth, the probe is withdrawn while the jetting fluid prevents the
hole from collapsing. By using water as a jetting medium, a
difference in hydrostatic head between the water filled hole and
the natural ground water table assists in the stabilization of the
cylindrical hole created by the probe. Generally, water should be
used when the natural material is fully saturated. Air is preferred
in cases where the existing soil is only partially saturated. The
use of compressed air prevents the creation of a vacuum beneath the
vibrating point when the vibrator is extracted. If desired, the
process of penetration may be repeated to insure that the hole
remains open over its entire depth and to insure that most of the
disturbed material has been flushed out or removed.
At sites where hole stability is not a problem, the probe may then
be removed and coarse granular backfill is dumped into the hole.
This backfill material generally consists of coarse gravel, crushed
stone or slag, sized 3/4 to 3 inches. It is possible to use sand,
and sand has been used in some applications as explained
hereinafter. The probe is then lowered again into the hole and
under its own weight with the assistance of vibration, it compacts
the backfill material. The probe has a shaped point which enables
it to displace the granular backfill radially into the soft in situ
soil. This process is generally repeated layer by layer until the
compacted column of granular material has been completed.
Repetition of this process and incremental feeding and compacting
produces a very dense granular column which is embedded with the
native cohesive soil. Depending upon the consistency of the natural
soil, columns of 3 to 4 feet in diameter are generally formed.
In the method illustrated in Ogawa U.S. Pat. No. 3,772,892, sand is
utilized instead of stone for the column. Nevertheless, these sand
columns will be referred to also as stone columns. In this regard,
while the term column tends to signify or imply that the columns
are rigid elements, they are not completely rigid, and give to a
certain extent under vertical loads as will be explained
hereinafter. Stone columns should thus be thought of as compacted
piles of granular material as opposed to stiff columns.
With the method disclosed in U.S. Pat. No. 3,772,892, the probe in
this instance consists of a thick-walled steel tube with a vibrator
on the top thereof. The probe is sunk into the subsoil to the
required depth, and if necessary, it is done so with the aid of
water and/or air jets. A sand plug at the lower end of the probe
prevents penetration of soil into the tube. At the required depth,
a known volume of sand is placed inside the tube and the tube is
withdrawn to some predetermined increment. During withdrawal of the
tube, sand is forced out of the tube by air pressure introduced at
the top of the sand. The process is repeated and the tube is
partially lowered again, forcing the freshly deposited sand into
the surrounding soil with the assistance of vibration, thereby
creating compaction. The ratio between the steps of extraction and
redriving governs the final cross section of the sand column. By
repetition of these steps, the sand column is gradually
constructed.
Other techniques suitable to form stone columns are also available,
such as a ram or weight falling in a cased hole through which
gravel or stone is introduced. This type of construction is
illustrated in U.S. Pat. No. 4,126,007 of Mars.
Generally, a plurality of such stone columns will be constructed in
a square or triangular grid pattern in the originally soft cohesive
ground, such that the ground is transformed into a composite mass
of vertical compacted granular cylinders with intervening native
soil. Such compacted granular columns are used not only for
structural foundations, but also for slope stability as a
preventive or corrective measure in either cut or fill slopes which
may otherwise cause either rotational or translational movement.
Such stone columns may also be utilized for embankment settlements.
The stone columns may be placed beneath embankments which are
underlain with soft cohesive material and thereby can limit and
hasten settlements, as well as improve stability. In many cases,
the controlling factor in construction is the post-construction
settlement caused by consolidation of underlying soft cohesive
soil. Stone columns installed in the soft soil not only decrease
the total amount of this consolidation settlement, but accelerate
the rate of settlement.
The stresses imposed on a vertically loaded compacted granular
column or stone column are similar to those imposed on a specimen
subjected to a triaxial compression test. Confining pressures
imposed by the in situ soil on the stone column act in a manner
similar to the cell pressure applied to the triaxial specimen. If
the vertical load on either the stone column or the triaxial
specimen is increased sufficiently, yield conditions (failure) will
occur. However, when this condition is reached in the stone column,
bulging occurs at the upper portions of the column, which imposes
radial displacement on the in situ soil. In this regard, an
illustration is shown in FIG. 8. If the native soil is loaded in
such a way that vertical deformation of the column and in situ soil
must remain equal, then this vertical deformation occurs
simultaneously with the radial deformation imposed by the bulging
column. The in situ soil deforms both vertically and radially, with
the effect that the radial stresses against the stone column
increase. This is illustrated by the horizontal arrows in FIG. 8.
Thus, vertical deformation of the stone column does not proceed
indefinitely, but reaches some equilibrium value which depends on
both the angle of internal friction of the column material and on
the compressibility and deformation characteristics of the soft
soil.
Since the lateral stresses of the in situ soil increase with depth,
stone columns resist vertical loads more efficiently at greater
depths. Vertical loads applied to stone columns by a structural
foundation or embankment impose the same vertical deflection on the
in situ soil as on the columns. The result is that the largest
vertical deformation of both the columns and the native soil occurs
immediately below the foundation and decreases rapidly with depth,
as is illustrated in FIG. 8. It has been shown that virtually all
of the settlement compression occurs in the upper 12 feet of the
stone column, even though the column itself may be 20 feet to 60
feet deep. Expected total settlement of the columns and the
surrounding in situ soil under load, might be expected to be as
much as 12 inches.
It should be borne in mind that this bulging effect of the stone
column when placed under vertical loads is a desirable effect. As a
means of explanation, if a rigid concrete pile in cohesive soil is
loaded, it settles developing end bearing and cohesive resisting
stresses. A compacted granular column or stone column similarly
develops these types of stress, as noted by the end bearing
stresses indicated by the vertical arrows at the bottom of the
stone column shown in FIG. 8 and by the cohesive resisting stresses
indicated by the smaller vertical arrows along the sides of the
stone column illustrated in FIG. 8.
However, the stone column as illustrated also bulges and is
therefore additionally supported advantageously by lateral stresses
exerted by the adjacent in situ soil as illustrated by the
horizontal stress arrows in FIG. 8. However, the problem with such
conventional stone columns is that settlement is limited to the
upper regions of the columns, undesirably causing excessive total
settlement. The present inventor discovered that this disadvantage
could be greatly alleviated if it were possible to transfer the
loads to a deeper level on the stone columns where the columns
operate more efficiently as afore-described. This would decrease
total settlement of the stone column under load, and would also
more uniformly distribute bulging of the column over the entire
depth of the column and in the deeper levels of the stone columns
where greater efficiency is attained as opposed to having the
column bulge only at its upper portions.
The stone column and the method of constructing a stone column in
accordance with the teachings of the present invention overcomes
the afore-mentioned disadvantages and provides the afore-mentioned
desirable objectives of transferring the loads to a deeper level on
the stone columns, where the columns operate more efficiently. In
its most basic form, the present invention comprises the
installation of a rigid central core in at least the upper portion
of the compacted granular or stone column. This can be accomplished
by driving a core of solid material downwardly and coaxially into
the stone column. This can also be accomplished by repenetrating
the finished stone column with a vibrating probe or the like to
make an elongated cavity in the stone column and then forcing a
rigid core into that cavity.
However, the preferred method is to penetrate a probe downwardly
into the compacted column of granular material to a predetermined
depth to form an elongated cavity in the compacted column and then
to withdraw the probe and fill the cavity thus formed in the
compacted column with cementitious grout to form a solid core after
hardening of the grout.
In addition, by providing the rigid central core in the granular or
stone column, the continuity of a water drainage pass through the
granular column to the ground surface is unbroken. This is
desirable in those situations where drainage through the column is
necessary. Conversely, if it is desired to block drainage from the
ground surface into the column, the entire composite column with
its central core may be grouted over to prevent intrusion of ground
water into the compacted granular or stone column. The rigid core
also acts to drastically increase the effectiveness of the stone
column when used to resist shear failures, as in slope
stability.
According to the teachings of the present invention, a compacted
granular or stone column is first constructed in the soil by any of
the afore-mentioned conventional techniques. This is accomplished
by penetrating a probe downwardly into the soil to be compacted to
a predetermined depth thereby forming an elongated cavity in the
soil. The probe is withdrawn from the soil cavity and at least a
portion of the cavity is backfilled with granular material such as
crushed stone, gravel or sand, and this granular material is
compacted in the cavity to form a compacted column of granular
material.
The stone column thus having been formed in the conventional manner
is then reconstructed in accordance with the teachings of the
present invention by penetrating a probe downwardly into the
compacted column to a predetermined depth, thereby forming an
elongated cavity in the compacted column itself. The probe is
withdrawn from the cavity formed in the compacted column, and the
cavity thus formed in the compacted column is filled with
cementitious grout to form a solid core after hardening of the
grout.
While it may be possible to merely pour the grout into the cavity
thus formed in the compacted column after withdrawal of the probe,
the preferred method is to inject grout into the cavity at the
lower end of the probe while the probe is simultaneously being
withdrawn from the cavity formed within the compacted granular
column. The grout is injected into the cavity at the lower end of
the probe in predetermined quantities metered in synchronization
with the rate of withdrawal of the probe from the cavity. One way
of accomplishing this is with the use of a positive displacement
grout pump, the pump output of which is regulated with the rate of
withdrawal of the probe from the cavity.
The injection of grout into the cavity beneath the probe being
withdrawn will generally be regulated such that the cavity being
formed beneath the probe as it is being withdrawn is continually
filled with grout. In fact, it will be found desirable in some
circumstances to continually fill the cavity beneath the retracting
probe with the grout in the cavity being subjected to continuous
pressure by injecting quantities of grout into the cavity at a rate
slightly faster than the rate of withdrawal of the probe, such that
the grout within the cavity is always under pressure, so that the
grout will be forced into the interstices of the walls of the
cavity.
The probe utilized for initially forming the cavity in the soil and
also for forming the core cavity in the stone column itself is
preferably of the type which can be laterally vibrated. This
greatly assists in forming the cavities and also in compacting the
granular backfill. After the initial cavity is formed in the soil,
the vibrating probe is usually withdrawn and a predetermined amount
of backfill is inserted into the soil cavity. The probe is then
reinserted into the cavity, and into this quantity of backfill, to
compact the granular material under the load of the probe and with
the lateral vibration of the probe. This step is then successively
repeated so that the compacted stone column is built layer by
layer.
After the stone column is completed, the laterally vibrating probe
is then reinserted into the compacted column to form the core
cavity for injection of grout. When withdrawing the probe while
simultaneously injecting grout into the bottom of the cavity, it
may be found desirable under some situations to continue lateral
vibrations of the probe in order to work the grout into the
interstices of the core cavity.
When penetrating either the native soil to form the initial soil
cavity or when penetrating the stone column to form the core
cavity, it is also desirable to jet a fluid under pressure, such as
compressed air or water, from the bottom of the probe to assist in
penetration.
If it is desirable not to have water surface drainage seep into the
stone column, then the stone column is only built up to a
predetermined level in the cavity formed in the soil and then after
the grout core in the upper portions of the stone column have been
formed, the remaining top portion of the soil cavity is completely
filled also with cementitious grout such that it covers the entire
top of the compacted column after the grout core has been formed
therein.
The preferred probe to be utilized in constructing the stone
columns of the present invention comprises an elongated vibrator
probe housing with means to vertically raise and lower the housing
such as a crane. Vibrator means are provided in the housing for
vibrating the probe housing laterally. Generally, such vibrators
will consist of eccentric weights which are rotated. A conduit is
secured to the probe housing and extends for at least substantially
the entire length of the housing, and opens at its bottom end
adjacent the bottom of the probe housing. Means is then provided
for providing a cementitious grout such as a grout hopper, and a
pump is connected to the upper end of the afore-mentioned conduit
and to the grout supply to pump grout in metered quantities
downwardly through the conduit and out the bottom opening at the
bottom of the probe.
When it is desired to synchronize the pump output with the rate of
withdrawal of the probe from the core cavity formed within the
stone column, this can be accomplished by a detector which detects
the rate that the probe is withdrawn or raised, and accordingly
regulating the output of the grout pump. For example, the means to
raise and lower the housing may include a wire rope and sheave
combination and a detector may be positioned in relation to the
sheave to detect the degree of rotation of the sheave when the
housing is being raised to determine the rate that the housing is
being raised for metering the grout ejection or output of the grout
pump.
When the grout conduit is not being used to pump grout, it is
desirable to continually pump water through the conduit when the
probe is being utilized to penetrate the soil or the stone column
in order to prevent the outlet of the grout conduit from being
plugged.
Additional water jets are also desirably positioned at the bottom
of the probe housing for assisting downward penetration of the
probe into the soil.
The probe housing may be effectively elongated by the addition of
extension or follower tubes which interconnect and are interposed
between the top of the probe housing and the means to vertically
raise and lower the probe housing. This not only effectively
lengthens the probe as required, but also provides a means for
regulating the amount of weight of the resultant probe for soil
penetration.
Other objects and advantages appear in the following description
and claims .
The accompanying drawings show, for the purpose of exemplification
without limiting the invention or the claims thereto, certain
practical embodiments illustrating the principles of this invention
wherein:
FIG. 1 is a diagrammatic view in front elevation illustrating the
probe of the present invention.
FIG. 2 is an enlarged view in side elevation of the upper portion
of the probe illustrated in FIG. 1 showing the wire rope and sheave
in combination with a detector for detecting the rate of rotation
of the sheave.
FIG. 3 is a diagrammatic view in side elevation illustrating the
probe of FIG. 1 as being carried by a crane and being further
connected to a grout supply in order to carry out the method of the
present invention for constructing compacted granular or stone
columns.
FIG. 4 is a diagrammatic view in side elevation illustrating the
probe of FIG. 1 in the initial step of penetrating soil to
eventually construct a stone column.
FIG. 5 is a diagrammatic view in side elevation illustrating the
second step in constructing a stone column by utilizing the probe
shown in FIG. 1.
FIG. 6 is a diagrammatic view in side elevation illustrating a
continuation of the method step illustrated in FIG. 5 toward
construction of a stone column.
FIG. 7 is a diagrammatic view in vertical cross section
illustrating a stone column constructed in accordance with the
sequence of method steps illustrated in FIGS. 4, 5 and 6.
FIG. 8 is a diagrammatic view in vertical section of the stone
column illustrated in FIG. 7 after a vertical load has been applied
thereto.
FIG. 9 is a diagrammatic view in partial vertical section
illustrating the stone column of FIG. 7 as being repenetrated by
the probe illustrated in FIG. 1 to form a core cavity in the upper
portion of the stone column.
FIG. 10 is a diagrammatic view in partial vertical section
illustrating grout being injected into the central cavity formed in
the stone column as the probe is being withdrawn.
FIG. 11 is a diagrammatic view in vertical section illustrating the
stone column of the present invention with the grout core as
constructed in accordance with the method teachings of the present
invention.
FIG. 12 is a diagrammatic view in vertical section illustrating the
composite stone column of FIG. 11 with a modification wherein the
entire upper portion of the composite column is grouted over to
prevent seepage of surface water into the column.
FIG. 13 is a diagrammatic view in vertical section illustrating the
composite stone column of the present invention as shown in FIG. 11
after having a vertical load applied thereto.
The method of the present invention is described in conjunction
with the earth probe illustrated in the drawings. While this
particular probe has novelty in and of itself, and is part of the
subject matter of the present invention, nevertheless, with regard
to the method of the present invention for constructing compacted
granular or stone columns in soil, it should be borne in mind that
other types of probes may be employed as hereinbefore
described.
Referring first to the vibrator probe as illustrated in FIG. 1, the
vibrator probe is basically similar to the type of vibrating probe
illustrated in Steuerman U.S. Pat. No. 2,667,749 with some novel
improvements. The vibrator probe 10 is comprised of an elongated
probe housing 11 containing means for vibrating the probe housing
laterally. This means in turn comprises electric or hydraulic motor
12 which rotatingly drives eccentric weights 13 and 14 through
coupling 15. Eccentric weights 13 and 14 are coupled together as
indicated to drive in unison and are mounted in bearings within
housing 11. A series of follower or extension tubes 16 are
connected to the top of probe housing 11 to effectively elongate
and add weight to probe 10 for soil penetration. In fact, the
weight of extension tubes 16 may be varied to gain the desired
penetration effect. Any number of extension tubes may be added to
the probe, depending upon the particular application or the depth
of the stone column to be constructed.
The vibrator probe 10 is vertically raised and lowered by any
conventional means such as the crane 17 illustrated in FIG. 3. In
this regard, a conventional wire rope 18 and sheave 19 are
employed.
Referring to FIG. 1, water feed lines 20 extend down the side of
extension tube 16 and probe housing 11 and exit at the bottom of
the probe at lower water jets 21. Referring also to FIG. 3, water
under pressure or air under pressure is applied to these conduits
20 in a conventional manner via the flexible feed pipe 22. The
water or air under pressure which jets from the bottom of the probe
at 21 is thus utilized to assist in penetrating the probe
downwardly into the earth. Water or air jets are also provided at
positions 23 on probe 10 as illustrated in FIG. 1 for reasons which
will be discussed in greater detail hereinafter. Valving is
provided such that water jets 21 and 23 may either be operated
independently or simultaneously.
Eccentric weights 13 and 14 on the lower part of the probe are
driven by the electric or hydraulic motor 12 which usually operates
in the range of 3,000 revolutions per minute at 50 Hertz to 1,800
revolutions per minute at 60 Hertz to create vibrations in a
horizontal plane.
The probe housing 11 is connected by a special elastic coupling 25
to follower tubes 16 in order to dampen vibrations which would
otherwise be imparted to follower tubes 16. The total weight of the
probe is adjustable by the addition of heavy or light weight
follower tubes 16 which can produce a total weight of approximately
4 to 8 tons for a 45 feet long probe. All electric cables or
hydraulic hoses and water or air hoses are connected to the
uppermost extension tube as illustrated in FIG. 3, and the
electrics or hydraulics are supplied through hydraulic conduit or
electric cable 26 from a conventional hydraulic or electric power
source.
While compressed air may be utilized, for description purposes the
use of water as the jetting fluid will be described. The lower set
of water jets 21 on the probe tip aid in probe penetration while
the upper set of water jets 23 assist in the removal of displaced
cohesive material which lies within the probe path.
While the probe 10 is illustrated as being supported by a
commercial crane 17, special supporting rigs may be substituted or
added (not shown) which can exert a downward hydraulic thrust to
assist in forcing the probe 10 into the ground. Other additional
supporting equipment might normally consist of a high capacity-high
pressure water pump, a portable energy source to provide power to
the motor 12, and a front end loader to feed required backfill
material into the cavity to be formed in the soil as hereinafter
described.
Fins 27 are circumferentially positioned about the bottom of probe
10 in order to prevent rotation of the probe on downward
penetration into the soil.
Probe 10 differs from the conventional vibrator probe by the
addition of a grout feed pipe or conduit 28, which extends for
almost the full length of the probe as is the case with the water
conduit 20. Grout conduit 28 terminates or exits into the nose cone
cavity 29 at the tip of probe 10. In order to provide cementitious
grout for feeding into grout conduit 28, a grout hopper 30 (FIG. 3)
is provided. Grout hopper 30 is further provided with positive
displacement grout pump 31 which feed forces grout from hopper 30
into conduit 28 to pump grout in metered quantities downwardly
through conduit 28 and out of nose cone cavity 29 when pump 31 is
energized.
A metering means is provided for controlling the operation of pump
31 in synchronization with the rate that housing 11 is raised by
the crane in order to eject grout through the nose cone tip 29 of
the probe in predetermined metered quantities relative to the rate
that housing 11 is being raised. This metering means includes the
use of magnetic detector 32 which is electrically connected to the
energization or power supply system for pump 31 via conductor 33.
Referring with particular reference to FIG. 2, sheave 19 is
provided on its side face 35 with a plurality of circumferentially
spaced permanent magnets 36. When crane 17 is raising probe 10 and
detector 32 is switched to its on mode, detector 32 will detect the
degree of rotation of sheave 19 as housing 11 is being raised by
counting the passages of permanent magnets 36 on rotating sheave
19. Detector 32 accordingly sends out electric pulses which in turn
correspondingly energize positive displacement pump 31 in order to
meter grout ejection at nose cone 29 of the probe 10 in
predetermined metered quantities relative to the rate the housing
11 is being raised.
The method of constructing a compacted granular or stone columne in
soil to increase load-bearing capacities thereof, together with the
resulting compacted granular or stone column of the present
invention are discussed hereinafter in relation to the remaining
Figures. Construction of the compacted granular or stone column is
described with the utilization of the vibratory probe disclosed in
FIGS. 1 through 3, even though other types of probes may be
utilized to carry out the construction method for constructing the
compacted stone column of the present invention.
In order to construct a compacted granular or stone column in soil
according to the teachings of the present method, a conventional
compacted granular or stone column is first constructed in any
conventional manner as previously described. For the purpose of
illustration, construction of a stone column will be described
using the probe 10 illustrated in FIGS. 1 through 3.
Referring to FIG. 4, the probe 10 is penetrated into the soft
cohesive soil 40 to a predetermined depth under its own weight and
with lateral vibrations as indicated by arrow 41 and with the
assistance of the jetting fluid 21 which may be water or compressed
air, but in this instance, it is water under pressure. During the
penetration process, the soil immediately surrounding the vibrator
or probe is disturbed or remolded to a nominal extent. The
disturbed material is flushed from the hole by water from jet 21
and also from the action of jet 23. After the probe has penetrated
downwardly to a predetermined depth to form the elongated cavity 42
in the soil 40, a certain amount of the soil 40 will be somewhat
more compacted than in its natural state around the cavity 42 as
indicated at 40' in FIG. 5.
After the elongated soil cavity 42 is formed, the probe 10 is
withdrawn and a layer of backfill in the form of granular material
43 is dumped into the soil cavity 42. This granular backfill is
disclosed here as coarse granular material such as crushed stone,
gravel or slag, sized from 3/4 to 3 inches in diameter. As
illustrated in FIG. 5, the probe 10 is then lowered into the cavity
42 under its own weight and with assistance of vibrations as shown
at 41, the backfill 43 is repenetrated and compacted. The process
is continually repeated as illustrated in FIG. 6, and the probe
displaces the granular backfill radially into the soft in situ soil
40 thereby building layer by layer and expanding the stone column
43 as illustrated in FIG. 6. The process is repeated until a
completed conventional type compacted granular or stone column is
attained as illustrated in FIG. 7.
FIG. 8 illustrates the stone column 43 of FIG. 7 after the stone
column has been vertically loaded as indicated by load vectors 50.
Under such vertical loads, the stone column 43 will yield, and
bulging occurs at the upper portion of the column as indicated at
51. The in situ soil 40 deforms both vertically and radially with
the effect that the radial stresses indicated by vectors 52 against
the stone column 43 increase. Thus, when the stone column 43 in a
cohesive soil 40 is loaded, it settles, developing end bearing
stresses indicated by vectors 53 and cohesive resisting stresses
indicated by vectors 54, and the stone column also bulges at the
upper portions 51 so that it is supported by lateral stresses 52
exerted by the adjacent in situ soil.
However, as previously pointed out, since the lateral stresses of
the in situ soil increase with depth, the stone column 43 resists
vertical loads more efficiently at greater depths. However, the
conventional stone column 43 illustrated in FIG. 8 under vertical
load has its largest vertical deformation of both the column and
the native soil immediately below the foundation level, and this
deformation decreases very rapidly with depth. It is thus an object
of the present invention to produce a system which will transfer
loads to a deeper level on the stone column 43 where the column
operates more efficiently.
Thus, in order to construct a compacted granular stone column in
accordance with the teachings of the present invention, the
compacted granular or stone column illustrated in FIG. 7 is further
modified by repenetrating the probe 10 with water flow as before,
and in addition, water is also continually flowed through grout
feed conduit or pipe 28 during this and all penetrating of the soil
or stone column in order to prevent clogging of the grout pipe and
nose cone cavity 29. In this manner, the probe is thus repenetrated
into the completed stone column 43 to a predetermined depth. As
just one example, the entire depth of the column might be 40 to 60
feet, and the probe 10 might be repenetrated to a depth of, say, 12
feet. This depth will vary with construction conditions. In this
manner, an elongated cavity 56 is formed in the compacted column
43. When the probe 10 repenetrates the stone column 43, it
self-centers within the stone column.
After forming the core cavity 56 within the stone column 43, the
probe 10 is withdrawn and grout is fed simultaneously at a measured
and synchronized rate as previously described, and as illustrated
at 57 in FIG. 10, in order to fill the core cavity with
cementitious grout 57. As indicated by dashed arrow 41', the
vibrator may or may not be operating at this time, as desired.
Also, as previously described, the grout 57 may be injected at a
rate slightly greater than the rate of withdrawal of probe 10 in
order to inject it into the bottom of the core cavity 56 under
pressure, so that the grout 57 will work its way into the
interstices of the stone of column 43. This is also assisted by the
vibrations of the probe 10.
The completed composite and compacted granular or stone column of
the present invention is illustrated in FIG. 11 having the upper
grout core 57 which is hardened or cured into a rigid central
core.
As an alternative construction procedure, a separate probe could be
utilized to perform only the grout operation, instead of using the
same probe for penetration and compaction in addition to grout
feed.
As previously described, the positive displacement grout pump 31
will inject grout into the cavity 56 at the lower end of probe 10
in predetermined quantities metered in synchronization with the
rate of withdrawal of probe 10 from cavity 56 by means of the
detector 32 which activates grout pump 31 as previously described
in conjunction with FIGS. 2 and 3. As an alternative, a laborer
could manually operate a positive displacement pump which pumps a
measured amount of grout with each stroke, whereby he would apply
one stroke for each calibration marked on the side of the probe 10
on the follower tubes 16 as the tube is withdrawn. The amount of
grout ejected during each stroke would be adjustable to provide the
desired grout core diameter.
By grouting only the central core of column 43, continuity of the
drainage path downwardly through stone column 43 to the ground
surface is maintained. However, if it is desired to block drainage
from the ground surface into the stone column 43, the grout can be
applied over the entire composite column cross section as
illustrated in FIG. 12. To accomplish this, the stone column 43 in
the first instance is not constructed all the way to the ground
surface as illustrated in FIG. 12.
When the composite column of the present invention, such as
illustrated in FIG. 11, is vertically loaded, the results are
illustrated in FIG. 13. By comparing this Figure to FIG. 8, it can
be seen that the result is that the loads are transferred to a
deeper level in the column 43 where the column operates more
efficiently due to the effect of rigid core 57. This causes the
column to settle less and further, more uniformly distributes the
lateral stresses 52' over the entire, or a greater portion, of the
column 43 and at lower depths in the column as opposed to the very
pronounced bulge only at the upper portion of the column
illustrated in FIG. 8. Thus, the lateral stresses 52' act lower on
the column providing a composite stone column which can resist
vertical loads much more efficiently. In addition, the grouted core
column of the present invention drastically increases the
effectiveness of the stone column to resist shear failures, which
is a very important factor for such stone columns when they are
used in sloped stability applications.
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